[test only] Improve `filesIn` test-helper function

[d-ronnqvist]

Bug/issue #, if applicable:

Summary

This is a test-only change that improves the flexibility of the filesIn integration test-helper in the integration tests.

This PR is in preparation for another change. Whenever I've asked people about how they that I structure my PRs they've said that they prefer multiple smaller single-purpose PRs. I'm trying to do just that with this PR. The purpose of this change is to support writing a new test for a follow up PR that intercepts and customizes the --output-dir argument.

Dependencies

None.

Testing

Run bin/tests.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• [x] ~Added~ Updated tests
• [x] Ran the ./bin/test script and it succeeded
• [x] ~Updated documentation if necessary~ Not applicable.

[d-ronnqvist]

@swift-ci please test

[d-ronnqvist]

@swift-ci please test

[d-ronnqvist]

@swift-ci please test